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1. 02:26 PM - Re: Re: OV protection circuit design (Robert L. Nuckolls, III)
2. 06:59 PM - Re: Re: OV protection circuit design (Art Zemon)
Message 1
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| Subject: | Re: OV protection circuit design |
This particular thread germinated from a query about the selection
of materials for EXTENDING a fuse-block bus up to a 5A breaker
on the panel. Normal operating currents flowing in this feeder
are not significant to the selection of components. The design
goal is to provide hard-fault protection for a BUS EXTENSION
such that an expected event that opens the 5A breaker will
not open the protection at the bus.
The N811HB accident had root cause in a failure to conduct the
necessary evaluation of components. Here's a video demonstrating
a critical comparison of fuses vs. breakers:
https://tinyurl.com/r6bakrp
Another video demonstrating a consequence of failure to exploit
that comparison:
https://tinyurl.com/s4bfb3m
>Electrical circuits require two types of protection, short circuit and
>overload. A short circuit occurs when two conductors touch each other.
>The current will be very high. An overload occurs when the load draws
>more current than it was designed for. Fuses protect against short circuits.
>Circuit breakers protect against overloads. Although sometimes only one
>type (fuse or breaker) is used to protect a circuit against both
types of faults.
>In the case of the alternator over-voltage module shorting to ground, we
>are dealing with a short circuit. We want the circuit breaker to
trip, but not
>the fuse to blow.
>I would have suggested using a 35 or 40 amp fuse if it would fit
into the fuse block, but it won't.
But a 30A will fit and has been tested for the application
described in the
drawing. By the way, you CAN buy ATC/ATO 40 fuses. I just discovered
this a few days ago. I think I'll upsize the recommended feeder extension
fuse to 40A.
https://tinyurl.com/tqg7bqb
>Normally we choose the fuse sized based on the wire size.
This is an over-simplification. The vulnerable feature of any wire
is generally not the wire itself but its insulation. Looking through
the wire catalogs we find a constellation of products with a range
of TEMPERATURE ratings combined with other qualities like abrasion
resistance, flexibility, chemical resistance, fire ratings, etc. etc.
The continuous operation design load for a wire is based
on temperature rise in wire due to loading, hotest ambient
expected and the rating for that wire's INSULATION. Of
course, there is the secondary but rare consideration
for voltage drop.
Selecting protection for any feeder is an engineering study
to meet design goals. Yeah, AC43-13 and similar documents offer
conservative guidelines that address the majority of your design
decisions . . . but as mentioned earlier, these are not
engineering texts. Information proffered does NOT go to full
understanding.
>But in this case, we are not dealing with a continuous load. We are
>dealing with a short circuit that only lasts a few seconds at most.
Actually, tens of milliseconds
>It takes time for a wire to heat up. The circuit breaker will trip
>before that happens. Using smaller wire will add resistance to the circuit,
>thus helping to limit the maximum current and helping to prevent the
>fuse from blowing before the circuit breaker trips. Using 12 AWG wire
>in this situation is actually counter productive because the current
>could be high enough to both blow the fuse and trip the breaker.
Not so . . . which I will explain
Most of the circuit protection found in vehicular DC power systems
are of the 'heating' variety. A calibrated, melting element within
an enclosure makes up a fuse . . . a thing that warms up in response
to a thru-current until the element melts, collapses and the
circuit is broken. Similarly, a mechanical switch spring loaded
to open up is held in a latched-closed condition by a mechanism
that opens the latch based on temperature rise in a heating element.
While all such heat-operated devices have a current 'rating',
that number is but a small piece of the story. Each class of device
will have a dynamic (i.e. current/time dependent) quality significant
in selecting the device to do a task. I chose 14AWG in the bus-to
-breaker segment so as to minimize trip time for the breaker. In
typical crowbar ovm systems, the response time from trigger to trip
is on the order of 10 milliseconds. But sizing this segment
was as much 'seat of the gray matter' as it was a calculated
selection.
As mentioned above, our critical design consideration is
evaluate the DIFFERENCE between the upstream and downstream
protection devices.
For example: You may have some appliance that is fitted with a fuse.
Perhaps internally or part of the line-cord plug. Then there are
breakers in your breaker-box. The transformer on the pole behind
your house is fitted with a fuse in the primary supply feeder. High
voltage feeders to your transformer and that of your neighbors
will also be fitted with fuses. As we move toward the source
in the power distribution systems, there will be protective devices.
Each one carries more power as you move up the distribution chain.
The fuse in your household electro-whizzy is expected to operated
without opening the 15 or 20A breaker in your breaker-box.
Similarly, each feeder breaker is designed to open without
tripping the main breaker. The fuse on the pole outside will
open without tripping anything upstream thus only you and
your immediate neighbors are in the dark. So goes the design
task all the way up to the turbine-driven alternators that
effect the large scale, mechanical-to-electrical power conversion.
For this thread, a practical perspective says that if all wires along
the subject BUS EXTENSION were 20AWG, the risk for failure to perform
would be essentially zero.
>I would not use 12 AWG because it is too big and hard to work with.
>It might even damage components that are not designed for large wire.
M22759, 12AWG wire is made up of 37 strands of 28AWG wire
. . . quite flexible and user friendly . . . not necessary
to avoid if you really need its capabilities.
>I would rather use a fuselink, like Charlie said, than use #12 wire.
Fusible links were added to our toolbox in the early days
of fuse-blocks. Fusible links had a ring terminal on one
end . . . this could bolt down on the fuse-block's feeder
terminal. A FAT ATC/ATO fuse (30+ amps) can be used
to tap the bus with a fast-on terminal. For our purposes
the two techniques are technically interchangeable; The
fuse/fast-on being simpler to fabricate. The actual sizes
for the wires is nearly insignificant . . . it's
all about the characteristics of TWO protective devices
IN SERIES.
I picked up a handfull of ATC30 fuses and a few ATC40
fuses yesterday. I need to dig around in the test
equipment to find the 300A hall-sensor I used for
crowbar ovp performance studies some years ago.
I'll revisit the current vs trip characteristics of
a 5A mini breaker with ATC30 and ATC40 fuses. It's
a quick experiment to run and document if you've
got the tools . . . now . . . if you were a 300A
full scale HE sensor, where would you be hinding?
Bob . . .
Message 2
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| Subject: | Re: OV protection circuit design |
Bob,
Those are very instructive videos. Thank you for sharing them. I will be
passing them on to some friends.
-- Art Z.
On Sun, Mar 8, 2020 at 4:40 PM Robert L. Nuckolls, III <
nuckolls.bob@aeroelectric.com> wrote:
> This particular thread germinated from a query about the selection
> of materials for EXTENDING a fuse-block bus up to a 5A breaker
> on the panel. Normal operating currents flowing in this feeder
> are not significant to the selection of components. The design
> goal is to provide hard-fault protection for a BUS EXTENSION
> such that an expected event that opens the 5A breaker will
> not open the protection at the bus.
>
> The N811HB accident had root cause in a failure to conduct the
> necessary evaluation of components. Here=99s a video demonstratin
g
> a critical comparison of fuses vs. breakers:
>
> https://tinyurl.com/r6bakrp
>
> Another video demonstrating a consequence of failure to exploit
> that comparison:
>
> https://tinyurl.com/s4bfb3m
>
>
--
https://CheerfulCurmudgeon.com/
*Sooner meet a bereaved she-bear than a fool with his nonsense. *Proverbs
17:12
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